Late Quaternary Activity And Geometric Complexity Of The Laohu Shan Section And Its Adjacent Area

The Haiyuan fault is one of the most important localized deformations in accommodating Indo?Asian convergence,which also includes the Altyn Tagh,Kunlun,and Xianshuihe strike?slip faults.It is responsible for two extremely devastating earthquakes,including the 1920 M 8.3 Haiyuan earthquake and the 1927M 7.7 Gulang earthquake,however,there is a²60-km‘seimic gap'between the two strong earthquakes,where bears no historical record of a large earthquake in the past several centuries.It has been considered has rather high potential seismic hazard.Therefore,intensive study of the deformation,fault behavior of this section is not only the key to understand the deformation mechanism of the northeastern Tibet Plateau,but also an important basis for analyzing strain distribution and seismic hazard evaluation.The Laohu Shan section of the Haiyuan fault is located in the‘seismic gap',and is connected to the west end of the 1920 Haiyuan rupture by a small left-lateral step-over,the Jingtai pull-apart basin.This section,with its relatively straight fault trace,exists numbers of well-preserved displaced geomorphology marks,such as those offset gullies,ridges and a small number of terraces.The deflected terraces often offer excellent offset markers for the research of the slip rate.Previous work by Lasserre et al.?1999?obtained the slip rate of 12±4 mm/yr at two sites,Majia Wan and Xuanma sites located north of the Songshan village,which is higher than recent geodetically determined rates on adjacent fault sections.We re-analyze and re-evaluate the slip rates,benefiting from new high-resolution airborne LiDAR data collected at their two sites,located north of the Songshan village.Based on this data,we revise field-mapped offset constraints,and augmented published age control with additional Quaternary geochronology.At the Majia Wan site,we document a sinistral displacement of 130±10 m of the upper edge and 93±15 m of the lower edge of T1/T2 terrace riser,respectively.At the Xuanma wan site,the offset of a T4/T1'terrace riser is updated to be 68⁺³/_-10 m.Combining new and published geochronology,we assess the abandonment ages of T2 as 26.0±4.5 ka and T1 as 9445±30 yr.The age of T1'?7624±43 yr?given in Lasserre et al.?1999?is retained.These data suggest a slip rate between 5.0±1.0 and 8.9^+0.5/_-1.3 mm/yr,based on the upper terrace and lower terrace reconstruction of displacement accumulation,respectively,or 7.0±2.0 mm/yr,on average,since²6 ka.This average slip rate is similar in range with the geodetic loading rate and thus indicates no secular variation.Our re-evaluation supports that apparent slip rate discrepancies in northern Tibet possibly share a systematic bias due to the use of lower-terrace reconstructions to interpret the age of offset.We also suggest that it is quite necessary to deeply analyze the formation and evolution of these diaplaced marks for obtaining more reliable slip rate.The Zihong Shan fault?ZHSF?,with a fine continuity,cuts through the center of the Laolong Wan Basin.Its west end meets the eastern end of the Laohu Shan section of the Haiyuan fault at the west of the Mijia Shan.Then it extends to the vicinity of Wozitan with its strike turns from the northwest to the west,finally intersects with the1920 Haiyuan earthquake rupture near the Huangliangtan.Field mapping and high-resolution DEM shows that the fault is dominated by left-lateral strike-slip with reverse component.We observed a wide variety of displaced terraces,alluvial fans and gullies at four sites,including Dougouping,Yaoshuigou,Hongliutan and Shimen.A cumulative offset probability density distribution was calculated to analyze character of the left-lateral displacement.The result implies the minimum offset of 2 m,and the maximum of 51.6 m,with the peaks of 4.8 m,7.6 m,15.2 m and 18.7 m.According to the statistical result,we speculate the average coseismic offset of the ZHSF was³ m.Using the Quaternary geochronology obtained from the terraces of the Dougouping site,we limited the slip rate of the ZHSF,which is 1.4-2.6 mm/yr since¹1 ka.Combined with the main Haiyuan fault and two other northern branches,we estimate the slip rate of the Haiyuan fault along this section is 5.5-7.5 mm/yr.Based on the aforementioned results,with the statistics of the regional slip rates by the previous works in the Laohu Shan section and adjacent areas of the Haiyuan fault,the results indicate the average slip rate of the main part of the Haiyuan fault is 7±2mm/yr,with no evidence of Latest Quaternary secular variation.Excluding earlier studies that lacked rigorous geochronological constraints,the horizontal slip rate along the Haiyuan fault appears to slightly decrease eastward from the Jinqiang He?JQH?section to the 1920 Haiyuan earthquake rupture.From the perspective of a larger regional scale analysis as the Qilian-Haiyuan fault system,the strain originated from the Indian-Eurasa collision is transmitted to the Haiyuan fault,as well as distributed on the Qilian thrust fault and the arc-shaped distributed Gulang fault and Xiangshan-Tianjingshan fault to the north.The slip of the Haiyuan fault were also converted by the Liupanshan fault,the uplift and the Cenozoic sedimentary basin to the end of the fault.In summary,we believe that the block model used to explain the deformation of the Tibet Plateau cannot be completely represented by the absolute slip rates of these major boundary faults,except that it is more reliable to obtain as much as possible on large boundary faults.Collecting more well-constrained fault slip rate data across a system of faults in the Tibet Plateau would help to evaluate whether intra-block deformation is as important as slip on major faults,by showing strain accommodation by a small number of major faults or by a fault population without as much distinction of major and minor faults.